The present invention relates to a high pressure metal vapor discharge lamp, and more particularly to a high pressure sodium vapor discharge lamp with improved starting characteristics.
High pressure sodium vapor discharge lamps generally have a higher efficiency, i.e., a higher lumen output per watt, than high pressure mercury vapor discharge lamps, metal halide discharge lamps or the like. However, sodium vapor lamps require a specially designed ballast for starting and stable operation because a high starting voltage is needed. The expense of the special ballast hinders general use of high pressure sodium vapor discharge lamps.
A high pressure sodium vapor lamp has been developed which has in its outer jacket a discharge tube and a starting device comprising a heating filament and a thermally responsive switch. Such sodium vapor lamps can be started and stably operated with a ballast for a high pressure mercury vapor discharge lamp. In the starting operation, the thermally responsive switch is operated by the heating filament so that the switching voltage of the thermally responsive switch is converted into high voltage pulses by the induction of the choke coil of the ballast, and the high voltage pulses are impressed upon the electrodes of the discharge tube so that the lamp may be started. Accordingly, such a high pressure sodium vapor discharge lamp does not require any special external high voltage pulse generating device so that it can be used in place of a high pressure mercury vapor discharge lamp in lighting devices equipped with conventional mercury vapor lamp ballast while enjoying the advantage that an intensity of illumination may be attained which is twice as bright as that of a high pressure mercury vapor discharge lamp.
However, a high pressure sodium vapor discharge lamp having such a starting device in its outer jacket generates a pulse voltage as high as 4000 volts in the starting operation due to the action of the thermally responsive switch. When such high voltage pulses are generated, there is a possibility that a dielectric breakdown may occur between the choke coils of the ballast, between the socket and the screw base of the sodium vapor discharge lamp or between other points in the lamp circuit. This possibility is especially high in circuits of mercury vapor lamp lighting devices in which the insulation has deteriorated after years of use.
Particularly in a lamp circuit in which a ballast having a short circuit current of 0.9 to 1.7 amperes, such as a ballast for an 80 watt mercury vapor lamp, is used with 70 to 90 watt sodium vapor lamps, the high pressure sodium vapor discharge lamps have small size bases of the E26 or E27 type so that the possibility of dielectric breakdown occurring at the fitting between the base and the socket during a high voltage starting pulse is quite high. Hence, it is necessary to reduce the pulse voltage at the start. However, if the pulse voltage is reduced, the lamp cannot be started smoothly and the desired lamp operation cannot be attained.